Usually I like to write up on my blog the things that go well. Today, I’ll write about a mistake–one that meant my latest iTeadStudio PCB order was totally wasted. Sad for me, but hopefully you can learn from it!
The Problem: Air Wires
Like many people, I do my layout in Eagle, and I use the autorouter. Some folks dis the autorouter, but I find it quite useful, as long as you change the layout grid down to 10 mils. The big problem is that if the routing fails, it doesn’t tell you about it. Worse still, running a DRC pass doesn’t flag it as an error! It just leaves some un-routed connections as ‘air wires’. See the small inconspicuous yellow lines going straight between some pads? Of course, air doesn’t conduct electronic signals very well, so you can send off a set of boards this way, wait a month, and realize you’re out of luck,
The problem here was those extra holes under the IC socket. This is the footprint of my battery pack. The holes keep the autorouter from running wires there, and there’s no where else for them to go. So the autorouter fails. Sadly, those holes are actually useless, because screws won’t really fit under the IC socket. The only hole which will be populated with a screw is the hole that’s just off the end. The solution was simple: Just remove the two extra holes and re-route. And send off to iTeadStudio again (sigh).
Eagle definitely should flag air wires as errors during DRC!!
The Solution: Greenwire
Before just ordering a new set of boards, I still wanted to double-check that there isn’t some OTHER mistake on this board, so it’s necessary to build it out totally working and bring it up properly. The solution? Greenwire!!
A ‘green wire’ is essentially a ‘patch’ for your hardware, also known as a ‘blue wire’ in some cultures. This time around, FOUR such wires are needed. To figure out how to do this, I made a map, exporting the board to a PNG file and drawing the wires by hand in GIMP. This was especially important because it’s important to have no overlapping wires. When the unit is attached to a battery pack, overlapping wires will cause an uneven (and ugly) mounting.
This turned out so well that I actually built a handful of these units even with the green wires. It doubles the amount of time needed to assemble the unit, but it does mean not waiting another month (or more) to get the next spin of the PCBs.